1. Field of the Invention
This invention is related to the preparation and use of control samples designed to simulate blood having defined properties in clinical laboratory situations.
2. Background of the Invention
The field of clinical diagnosis has been rapidly expanding in recent years. Numerous new diagnostic techniques have been developed, and old techniques have been improved. Apparatus capable of making clinical measurements have also been an object of much development.
All of these laboratory methods and apparatus require periodic verification of their ability to perform properly. Verification is usually obtained using a control sample having a pre-determined property, typically a set value for the analyte concentration or other property being measured.
Control samples must have a number of properties in order to be effective. These include stability during storage times preferrably lasting several months or years, reproducibility, and ease of handling. When an analyte concentration is being measured, the control sample can typically be an aqueous solution of the analyte (if the analyte is stable in a aqueous solution) or a measured quantity of the analyte in the dry state to which water or another liquid is added. The manufacture of such control samples is relatively straightforward compared to complex biochemical control samples that are related to physical or biochemical properties of a complex substance such as whole blood.
A typical complex biochemical measurement requiring a complex control sample is prothrombin time coagulation testing, which requires the presence of complex plasma factors from the clotting cascade. Because of the difficulty in separating, purifying, and re-constituting such a complex sample, plasma is normally used as a control sample. This is satisfactory for the typical prothrombin time testing conducted in clinical laboratories, which is performed on an anti-coagulated plasma sample after reaction with calcium ion and a reagent. In the typical test, whole blood samples are collected by venipuncture into a citrate anti-coagulant. The citrate chelates the calcium ion, which is an essential factor in blood coagulation. The reagent added during the analysis contains excess calcium which neutralizes the excess citrate and restores the required level of calcium ion, allowing normal coagulation to occur so that a prothrombin time measurement can be made. Under these circumstances, a lyophilized citrated plasma control is sufficient for use as the control sample since the test does not depend on a physiological level of free calcium ion or the presence of the red blood cells in the sample.
However, lyophilized citrated plasma control samples are not sufficient for all types of available prothrombin time testing apparatus. U.S. patent application Ser. No. 762,748, filed Aug. 5, 1985, which is herein incorporated by reference, describes a simple device and method which requires the presence of red blood cells in a sample in order to determine prothrombin time. The device includes a capillary segment which acts as a pump, a reaction chamber at one end of the capillary, an inlet port for blood into the reaction chamber, and a vent at the other end of the capillary. The capillary and the reaction chamber provide for capillary flow due to surface tension and mixing of the assay medium with the reagent that is held in the reaction chamber. The reagent accelerates the coagulation process, which stops the flow of blood through the capillary. Blood flow is measured by the detection of the flow of red blood cells between a light source and a detector. Thus, any control sample must contain red blood cells or similar particles since it is the cessation of the motion of the particles that is detected by the instrument. Prior art plasma control samples are thus insufficient.
Fresh whole blood cannot be used as a control sample in such a device since clotting factors either deteriorate or are activated with the passage of time. Whole blood is therefore not sufficiently stable to allow its use as a control sample.
Accordingly, there is a need for a whole blood control sample for use in the apparatus described above and for use in other assays in which whole blood control samples would be advantageous.